In order to obtain accurate readings from a flowmeter it must be periodically calibrated by determining its characteristic, i.e., the constant of proportionality between the flow rate of the fluid flowing through the flowmeter and the response given by the flowmeter. In the case of a turbine flowmeter that develops electrical oscillations proportional in number to the volume of flow through the flowmeter, this characteristic, called the K-factor, is expressed in terms of the number of pulses generated by the flowmeter per unit volume of fluid passing through the flowmeter. The flowmeter characteristic is a function of the type of fluid, as well as the fluid temperature, pressure, and flow rate, and varies as the parts of the flowmeter wear in the course of use. An apparatus to determine the characteristic of a flowmeter while in an operating fluid system is called a prover.
A known technique for determining the characteristic of a flowmeter is to compare its response to that of a positive displacement prover connected in series with the flowmeter. Such a prover employs a piston that travels in a cylindrical chamber in synchronism with the fluid traveling through the flowmeter. By measuring the time interval required for the piston to travel a given distance through the chamber, and thereby displace a given fluid volume during a test run, an average flow rate can be calculated which is used to determine the flowmeter characteristic. U.S. Pat. No. 3,492,856 discloses a prover in which the piston has a passage through it and a poppet valve that seals the passage when closed, and permits fluid flow through the passage from one side of the piston to the other when open. The valve is closed during each test run, but open at other times.
In a positive displacement prover, leakage through the piston seals from one part of the cylindrical chamber to another can seriously affect the accuracy of the measurement. As a result, pistons have been developed which have two O-ring seals against the interior of the cylinder and an annular cavity between the seals. A fluid line connects the cavity between the O-ring seals to a container so that the leakage can be measured. See, e.g., U.S. Pat. No. 4,372,147 to Waugh. The existing passive leak detection systems, however, may give inaccurate leakage readings because leaked fluid may remain in the fluid line, avoiding detection. In addition, existing passive leak detection systems do not indicate leakage through the poppet valve seals and they do not indicate leakage while the prover is in operation nor sort test runs according to whether leakage occurred during the run. Finally, they do not automatically verify that the leak detection system is functioning properly.